Assessing the impact of the global subsea telecommunications network on sedimentary organic carbon stocks

The sequestration of organic carbon in seafloor sediments plays a key role in regulating global climate; however, human activities can disturb previously-sequestered carbon stocks, potentially reducing the capacity of the ocean to store CO2. Recent studies revealed profound seafloor impacts and sedimentary carbon loss due to fishing and shipping, yet most other human activities in the ocean have been overlooked. Here, we present an assessment of organic carbon disturbance related to the globally-extensive subsea telecommunications cable network. Up to 2.82–11.26 Mt of organic carbon worldwide has been disturbed as a result of cable burial, in water depths of up to 2000 m. While orders of magnitude lower than that disturbed by bottom fishing, it is a non-trivial amount that is absent from global budgets. Future offshore developments that disturb the seafloor should consider the safeguarding of carbon stocks, across the full spectrum of Blue Economy industries.

The work presented in this paper is original, and I am very supporting to this. Particularly when it involves the deep sea which is typically characterised by scarcity of data. One may argue that the results are of low importance because they show a relatively low impact of carbon disturbance, and a (really) unknown degree of Carbon remineralisation. However it is very important to consider all possible aspects of humans activities in the marine environment however small. Particularly if the environments affected are slow in recovery, as it is the case in the deeper parts of the ocean; many of them particularly those that tend to be Carbon hotspots (either for burial or remineralisation) are classified as vulnerable. Under this light the aim is sound and the principle of the work increases our understanding that whatever we do does actually impact the marine environment; this should be taken into account when plans for any type of ocean exploitation are proposed. Therefore I find the work pioneering, certainly a small but nevertheless good step to the right direction. The final suggestion that carbon disturbance and its potential remineralisation should be taken into account for similar activities (e.g. laying of cables for wind turbines), leading eventually to some sort of establishing carbon footprints for human activities in the sea, is certainly very topical and necessary. The limitations of the work are clearly explained, namely the coarse resolution of the data and model that estimates carbon contents as well as the even bigger question about remineralisation. The approach seems sound to me although some more clarity about the calculations would help. Some suggestions are made as annotated comments in the pdf file but apart from that I have no major objections for this to work to be published. Some refinements could be still made although I doubt that will affect the result greatly. The main value of the work, as I highlighted above is that it emphasizes the need for such work and provides a very good impetus to study the deep and even coastal oceans, including transitional environments such as coastal wetlands, with a higher intensity and better resolution.
Reviewer #3 (Remarks to the Author): The manuscript "What is the impact of the global subsea telecommunications network on sedimentary organic carbon stocks?" provides an important overview of a currently unrecognized source of anthropogenic disturbance on the seafloor and potentially impact that the installation of sub sea cables could have on the carbon stored with the sediments.
The authors estimate that between 2.82 -11.26 Mt of organic carbon has been disturbed to to cable installation in waters to a depth of 2000m. Though this is small amount of carbon it is important to understand and account for all anthropogenic seabed disturbance.
This work is important and required yet I believe the data currently available (as discussed in the manuscript) is not suitable to produce accurate or robust estimates of carbon disturbance and possibly remineralization at the global scale. Therefore I believe this manuscript would make a better perspective article highlighting the issues, our current understanding (or lack of) and the research questions that need to be answered to get a grip on the carbon impacts of subsea cable installation.
Below I have some technical questions and comments on the data used in this study: This study assumes a range of cable burial depths ranging between 0.5 and 2m (Line 87). The sedimentary carbon data used is from Atwood et al., 2020 is only to a depth of 1m, how do the authors extrapolate the data to a depth of 2m ? The Atwood et al., 2020 data is the only current data set the quantifies carbon in the top 1m of sediment but much of the data used in these stock estimates only represent the top 30-50cm of the sediments. The 1m carbon stocks in Atwood et al., 2020 are calculated from this data which potentiality results in a overestimation of carbon stored in the sediments, if this study further extrapolates the Atwood et al., 2020 data to 2m depth these errors will be significant.
Line 118: The authors use remineralization rate of between 20-60% taken from the literature. These values may not be appropriate for sediments globally. The Sala et al., rates are widely discredited in the literature. While the rates reported by Paradis and De Borger are much better they are geographically constrained (Mediterranean and North Sea) and there relevance at a global scale needs to be discussed further. In the preceding lines the difficulties of accurately estimating these rates are discussed I would consider if with the currently limitations the text should purely focus on the sediment/carbon disturbance and not the loss.
Annually it is estimated the sediments bury around 150 Mt of carbon it would be useful to assess potential recover times and highlight area more at risk (deep sea -slower accumulation rates). For example TAT-8 was the first fiber optic subsea cable installed in 1988 using this as a start date and 2020 as the date of the carbon map I would estimate that annually that between 0.09 and 0.35 Mt of OC per year is disturbed. This is of course crude but provides important context. This could be broken up into the different depositional areas to provide greater insights and potentially highlight the areas at greatest risk.
Ocean acidification is mentioned a few times as potential impact of disturbance of seabed organic carbon though true this could potentially happen, care must be taken to consider that the seabed holds large quantities of inorganic carbon that once disturbed would buffer against changes in pH for a significant period.
Again I think this manuscript is important for highlighting a currently unrecognized anthropogenic impact on the seabed but the lack of data at the global scale hinders accurate quantification of carbon disturbance and losses therefore I feel that the manuscript should be reworked into a perspective article.

Response to Reviewers
We thank the reviewers for their thoughtful reviews and we are pleased to receive such supportive reactions to our study, with three reviewers commenting this is "a well-written paper, likely in the interests of the general audience", raising " a potentially important and long-ignored source of the organic carbon disturbance and loss on the seafloor" (Reviewer 1), agreeing that "the work presented in this paper is original" and "pioneering" (Reviewer 2) and "provides an important overview of a currently unrecognized source of anthropogenic disturbance on the seafloor" (Reviewer 3).
We have made a number of changes to address the reviewer comments that include: -Adding further commentary on the fact that the global distribution of sedimentary carbon stocks is a model, based on relatively sparse, albeit >11,000 sampling points and that the model only extends to 1 m; hence (in the absence of another global model) we extrapolate the same concentration values down to 2 m (the maximum depth of cable burial assumed in this study). -Explaining how these uncertainties may affect our results, and highlighting the need for future studies (field and laboratory) to better constrain the disturbed volumes. -Reducing the focus on "lost carbon", given the uncertainties and reliance on few field studies that examined the effects of bottom trawling. We have now removed the values of carbon loss from Table 1 and instead focus on the disturbed sediment and carbon volumes alone. -Expanding upon uncertainties in remineralization rate across the global ocean as a result of varying biological, chemical and physical conditions in the Discussion, including citation of relevant modelling and field studies that provide further details on the complexity of the system. -Adding in further clarifications concerning the limitations of the global sedimentary carbon model (e.g. it does not include local Blue Carbon systems such as mangroves or seagrasses), modified language around "potential" enhanced ocean acidification etc.
Below are the responses to each individual reviewer comments, starting with their high-level comments followed by a point by point basis to their more detailed comments. Below the reviewer comments are in italics and our responses in bold. We refer below to line numbers in the 'clean' version of the manuscript. We agree that this study has the potential to set a new research agenda and primes future data acquisition and research that aims to provide greater spatial coverage and depth resolution of sedimentary carbon stocks, (as also recognised by Reviewer 2, see below). We were pleased that this reviewer recognises that we identify a "potentially important and previously-ignored source of organic carbon disturbance and loss" -this was our primary aim, as well as identifying opportunities to mitigate this potential loss in future.

Responses to Reviewer #1
We thank the reviewer for their positive appraisal of our work. As they correctly state, there remains a paucity of detailed sedimentary carbon data on a global basis; however, we consider that this is not a reason not to perform the analysis (which they also support). We use the most appropriate data that are available, but now provide an expanded commentary on the limitations of the existing data, and hence also of uncertainties that may propagate into our estimates. We have modified the text to ensure that we are more careful to highlight these propagated uncertainties, including the following revisions:  In relation to extrapolation of the Atwood et al. dataset from 1 to 2m below seafloor: Line 103 "In the absence of any global dataset that extends below one meter, we necessarily assume a similar concentration of organic carbon exists to a depth of two meters (i.e. the maximum depth of cable burial assessed here). We accept this may result in an over-estimated disturbed carbon stock for that lower meter, and this data gap clearly underlines a need for greater constraint by future studies."  In reference to speculative assessment of carbon loss: Line 133 "These remineralization rates were highest in areas affected by the greatest frequency of bottom trawls; however, as cable burial is a one-off activity, the highest remineralization rates are considered to be unlikely. Speculatively assuming the lowest loss rate (i.e. 20%) from these studies, would result in a cumulative loss of 0.144-1.17 Mt of previously-buried organic carbon on the continental shelf and 0.136-1.09 Mt on the continental slope (a total of 0.280-2.25 Mt globally). However, to date no study has specifically studied the effects of cable burial on carbon disturbance at field-scale and the whether findings from bottom trawling are truly applicable to cable burial remains unclear. Consequently, there remains considerable uncertainty in the fate of sedimentary organic carbon disturbed by cable burial" We also now also remove the carbon loss from Table 1 as it is a more speculative estimate (see also response to other reviewers).
 In further reference to the uncertainties in carbon loss: Line 155 "Previous studies have attempted to calculate a mean global oxidation rate; however, there is significant variability, due in a large part to controls exerted by ocean depth, deposition rate and primary productivity, resulting in large uncertainties52. The degradability of organic carbon, and hence remineralization rates, strongly depend on the physiographic environment and the associated chemical, biological and physical processes52-54. For example, regional differences in water column and sediment oxygen concentrations, and hence markedly different carbon remineralization rates may occur in different areas, such as coastal hypoxic zones that will feature very low remineralization rates58. The rate of reactivity can vary over at least four orders of magnitude in marine sediments worldwide34."  New text to highlight potential differences in carbon loss associated with different burial types: Line 147 "A particularly important control is likely to be the cable burial tool that is used, and the nature of the initial disturbance. In the case of ploughing and trenching, sediment typically settles quickly (particularly granular sediment, such as sand) and deposits close to the initial excavation site; in many cases immediately (fully or partially) backfilling the trench21. In such cases, the likelihood of remineralization will be reduced; however, in the case of jetting (which fluidizes the sediment), suspended plumes of fine (clay and silt-size) sediment may be more widely dispersed by ocean currents, taking days to settle and hence increasing the chances of remineralization21,49."  We also add the following text to the caption for Figure 5 "Note that this refers to the volumes of carbon potentially disturbed, but there remains large uncertainty concerning how much of that carbon will be remineralized and hence lost"  Conclusion emphasises this uncertainty and the need for future studies to provide greater constraint: Line 296 "This study presents the first assessment of the sedimentary carbon that may have been disturbed by cable burial, but the uncertainties in our estimates underline a pressing need for field and laboratory-based calibration studies to determine the fate of disturbed organic carbon. Such studies are essential to constrain organic carbon disturbance and loss across a wide range of water depths, and diverse physiographic and oceanographic settings, to quantify the true loss and vulnerability of sedimentary organic carbon to human activities.".

Responses to Reviewer #2:
The work presented in this paper is original, and I am very supporting to this. Particularly when it involves the deep sea which is typically characterised by scarcity of data. We were pleased that this reviewer recognised the import of the work and that our approach is sound and the limitations are clearly explained; however, as outlined in our detailed response to Reviewer 2 below, we have further strengthened the commentary on limitations. With regards to their request on some further clarity on the calculations, we have expanded the methods section, adding the following text to more clearly explain the methods: Line 396 "Calculating the length of fiber-optic telecommunications cables in the ocean The total length of submarine telecommunications cables was determined by summing the total length of all of the individually identified cable sections in a proprietary database provided for this project by Global Marine Ltd. This database details precise cable locations, including operational cables and those that have been decommissioned (out-of-service cables). Cross-checking this length against an open-access database of cable lengths (Telegeography: https://www.submarinecablemap.com/), indicates a difference of less than 3%, with a total length calculated from the Global Marine database of 1.82 x 10 6 , compared to 1.88 x 10 6 from Telegeography. Of the total length in the Global Marine database, 13.6% of the cable length (2.47 x 10 5 ) was reported to be out-of-service as of the December 2020. As the Telegeography database does not provide precise location information, we necessarily use the Global Marine database to calculate the length of cable that requires burial. An estimated 13.5% of the total length lies within Areas Beyond National Jurisdiction.
Calculating the volume of disturbed sediment and carbon along cable routes In order to calculate the volume of sediment disturbed by cable burial activities, we first determine the length of cables that are laid in water depths where burial is required. We use the 2022 GEBCO bathymetric map of the oceans (GEBCO, 2022) to determine water depths along each of the cable routes in the Global Marine database. We first excluded all cable lengths that lie in water depths >2000 m. We then differentiated by cable lengths that lie on the continental shelf, the continental shelf between to water depth of 1500 m, and between 1500 m and 2000 m (based on the World Seafloor Geomorphology map of GRID Arendal51. We make this differentiation because cables are typically buried to water depths of up to 1500 m, but in some regions (particularly the NE Atlantic) burial is also required to 2000 m water depth. In so doing, we aim to provide a conservative upper bound (i.e. including water depths of up to 2000 m). We then relate these cable lengths to the dimensions of the trenches excavated for cable burial, which provide upper and lower bounds for the potentially disturbed volume of sediment. Disturbed seabed area is derived by multiplying cable length by trench width (0.5-1.0 m), and then related to disturbed sediment volume by multiplying that value by trench depth (0.5-2.0 m). Finally, we relate the disturbed sediment volumes to the global modeled sedimentary carbon stocks of Atwood et al.2. We do this in two ways. First we simply base this on global average values of carbon/km 2 within the top 1 m below seafloor that Atwood et al. provide for the continental shelf and continental slope. Second, we use the mapped values of carbon/km 2 from the global model of Atwood (i.e. Figure 2B), extracting the values along each cable route to enable a more geographically-resolved calculation. Where we assume a burial depth scenario of 0.5 m, we half this value, and for a burial depth of 2 m, we double the value".
Some suggestions are made as annotated comments in the pdf file but apart from that I have no major objections for this to work to be published. Some refinements could be still made although I doubt that will affect the result greatly. The main value of the work, as I highlighted above is that it emphasizes the need for such work and provides a very good impetus to study the deep and even coastal oceans, including transitional environments such as coastal wetlands, with a higher intensity and better resolution.
We took the suggestions in the annotated document into account (see detailed response to Reviewer 2). We were also glad that the reviewer recognises the range of uncertainties in our final calculations should be a key driver for future research into sedimentary carbon across the global ocean, from coastal to deep sea settings. We hope that this study sets a new research agenda -and is implemented in policy recommendations across a range of offshore industries.

Response to detailed comments from Reviewer 2
Line 10 -Perhaps it is important to highlight the global marine sedimentary carbon storage here, i.e. similar or exceeding soil Carbon and/or 2 or 3 x atmospheric C.
There is insufficient space in the introductory abstract paragraph to add this comment. Given the fact that multiple studies present competing values, we feel that the statement in the opening sentence of the Introduction makes this point already: i.e. "Marine sediments are the largest store of organic carbon on Earth…" Line 32 -extra space Needs correction Space added as requested.
Line 51  Line 71 -It is a model really based on existing data which, although extensive, is nowhere near complete This is a very good comment. We have modified the text for clarity to say "We integrate a global database that documents the extent and locations of submarine telecommunications cables, with a model of organic carbon hosted in modern ocean sediments worldwide (that is based on interpolation between >11,000 sampling points)" Line 73 -Only a minor comment. It may be better to 'rationalise' these questions, into objectives 1a, b, 2a, b, 3 and 4. We considered making this change, but prefer to phrase these as questions rather than numbered objectives as we feel it reads better.
Line 124 -This is true and some information on type or state of organic matter would be good, although I am not expecting this to be easily available if at all.
The reviewer is correct. This information is not readily available; hence, we have expanded the text to reference the uncertainties and the need to understand regional to local biological, chemical and physical conditions in the environment. We have added reference to three relevant studies that expand on these points.
Line 153 "Second, organic carbon mineralization rates will depend on external factors. For example, not all organic carbon stored in sediments is labile, and may not be remineralized after disturbance33. Previous studies have attempted to calculate a mean global oxidation rate; however, there is significant variability, due in a large part to controls exerted by ocean depth, deposition rate and primary productivity, resulting in large uncertainties52. The degradability of organic carbon, and hence remineralization rates, strongly depend on the physiographic environment and the associated chemical, biological and physical processes52-54. For example, regional differences in water column and sediment oxygen concentrations, and hence markedly different carbon remineralization rates may occur in different areas, such as coastal hypoxic zones that will feature very low remineralization rates58. The rate of reactivity can vary over at least four orders of magnitude in marine sediments worldwide34." In relation to this and following points of discussion: The length of time that the disturbed sediment is exposed to oxic conditions is key. In a very rudimentary manner, without having done any work, it seems that the jetting method may be more influential in Carbon remineralisation that the other two as it will induce resuspension that may take time to settle. This would be particularly important in fine grain sediments that take long(er) to settle. So sediment type is another factor that would influence these processes. Is there any information on this (e.g. grain size or simply muddy vs sandy sediments)? If this was available even in some areas then a slightly better constraint on remineralisation may be possible, e.g. using the jetting method in muddy areas may be presumed to have the highest OM decomposition (or OC remineralisation) rates This is a very good point and we now discuss the different types of burial tools and how jetting may create suspended plumes that can be widely dispersed and take days to settle out (and hence increasing potential for carbon remineralisation in those suspended plumes) compared to other approaches that may result in shorter periods of exposure to the oxic conditions. Following jetting in fine grained sediments, turbidity can persist for several days, depending on the duration of the whole cable-laying process. For example, at the Nysted offshore wind farm (Denmark), one month was necessary to excavate 17,000 m 3 of sediment for a 10.3-km long, 1.3-m wide and 1.3-m deep cable trench. However, at any given location on a cable route, disturbance will typically persist from a few hours to a few days (Taormina et al., 2018).

Jørgensen
The following text is now included in the manuscript: Line 147 "A particularly important control is likely to be the cable burial tool that is used, and the nature of the initial disturbance. In the case of ploughing and trenching, sediment typically settles quickly (particularly granular sediment, such as sand) and deposits close to the initial excavation site; in many cases immediately (fully or partially) backfilling the trench21. In such cases, the likelihood of remineralization will be reduced; however, in the case of jetting (which fluidizes the sediment), suspended plumes of fine (clay and silt-size) sediment may be more widely dispersed by ocean currents, taking days to settle and hence increasing the chances of remineralization21,49."